1. Field of the Invention
The present invention relates to a beam irradiation apparatus for irradiating a target region with a laser beam and, particularly, to a beam irradiation apparatus mounted on a so-called laser radar, for detecting the presence or absence of an obstacle in a target region and a distance to an obstacle based on reflection light of a laser beam emitted to a target region.
2. Description of the Related Art
In recent years, a laser radar is mounted on a family car or the like in order to enhance safety during driving. The laser radar emits a laser beam to the front in the driving direction and detects the presence or absence of an obstacle in a target region and distance to an obstacle. Generally, the laser radar scans a target region with a laser beam and, based on the presence or absence of reflection light in each of scanned positions, detects the presence or absence of an obstacle in each of the scanned positions. Further, based on required time from a laser beam emission timing in each scan position to a reflection light reception timing, the distance to the obstacle from the laser radar in the scan position is detected.
To enhance detection precision of a laser radar, a target region has to be properly scanned with a laser beam, and each scan position of a laser beam has to be properly detected. As a laser beam scanning mechanism, a scan mechanism using a polygon mirror and a lens-driving-type scan mechanism for two-dimensionally driving a lens for scan are known.
On the other hand, as a method different from the scan mechanisms, a mirror-turning-type scan mechanism can be proposed. In the scan mechanism, a mirror is supported so as to be driven about two axes. The mirror is turned about each of the drive shafts as an axis by an electromagnetic drive force between a coil and a magnet. A laser beam is obliquely incident on the mirror. By two-dimensionally driving the mirror about each of the drive shafts as an axis, a target region is scanned in the horizontal and vertical directions with reflection light of the laser beam by the mirror.
In the scan mechanisms, scan positions of the laser beam can be detected by, for example, disposing a light source which emits a servo beam in addition to a light source which emits the laser beam and receiving the servo beam by a photodetector such as a PSD (Position Sensing Device) or the like. In this case, a configuration of making the servo beam displaced on the photodetector so as to trace a scan of the laser beam is disposed.
For example, the scan mechanism of the mirror rotating type can employ a configuration of disposing a servo mirror which rotates with a mirror to reflect the laser beam to the target region and making a servo beam incident on the servo mirror. Alternatively, a configuration of disposing a transparent member having a flat plate shape in place of the servo mirror, and making a servo beam incident on the transparent member can be used. In the case where the servo mirror is used, a servo beam reflected by the servo mirror is received by a PSD or the like. In the case where the transparent member is used, a servo beam passed through the transparent member is received by a PSD or the like. When the mirror rotates at the time of a scan with a laser beam, the servo mirror or the transparent member rotates in association with the rotation of the mirror, and the servo beam is displaced on the PSD. Based on an output signal from the PSD, the scan position of the laser beam on the target region can be obtained.
In the beam irradiation apparatus having the above-described configuration, a laser beam is emitted as a pulse beam to the target region at a predetermined timing. In this case, the power of the laser beam emitted toward the target region is remarkably higher than that of a servo beam incident on the PSD (by about 100,000 times). Consequently, when the laser beam is incident on the PSD as ambient light, an output signal from the PSD largely changes, and a problem such that an error is included in the position detection signal occurs. Normally, the wavelength of a servo beam is set so as to be different from that of a laser beam. As described above, the power of the laser beam is remarkably higher than that of the servo beam. Due to this, it is extremely difficult to completely block the PSD from the laser beam with a filter or the like. Even if a filter or the like is used, the laser beam exerts some influence on a position detection signal as ambient light, and a problem such that the precision of the position detection signal deteriorates occurs.